1. Field of the Invention
This invention relates to a process of preparing a microcellular foam by a high internal phase emulsion (HIPE) polymerization process and, more particularly, to a HIPE polymerization process for preparing a hydrophilic/oleophilic foam including polymerizing a water-insoluble monomer in a continuous organic phase, and polymerizing one or more water-soluble monomers and/or cross-linking a water-soluble polymer in an internal aqueous phase.
2. Brief Description of Related Technology
Foams are used as absorbent material, insulation, light weight structural articles, cushions and impact (or shock) absorbers. Hydrophilic microcellular foams prepared by HIPE polymerization have been of particular use as absorption media in a host of applications ranging from personal care, such as diaper and adult incontinence devices, to general fluid absorption for spills.
An emulsion is a dispersion of one liquid phase in another, immiscible, liquid phase. Among the many emulsions are oil-in-water and water-in-oil. In an oil-in-water emulsion, an oil phase is dispersed in an aqueous phase, while in a water-in oil emulsion a water phase is dispersed in an oil phase.
In general, essential ingredients in an emulsion polymerization include water, a water-insoluble monomer, an oil-in-water emulsifier capable of suspending oil (organic) droplets in a water (aqueous) phase, and a polymerization initiator, such as a free radical initiator compound (or compounds) which releases free radicals capable of initiating polymerization. HIPE polymerization processes typically include, in addition to the ingredients listed above, a water-insoluble comonomer, and a water-soluble electrolyte for the purpose of preventing the water-insoluble monomer and comonomer from dissolving or dispersing in the aqueous phase.
HIPE polymerization processes have proven to be an excellent method of preparing low density foams for use as absorption media, and are described, for example, in U.S. Pat. No. 5,387,207 to Dryer et al. and U.S. Pat. No. 5,331,015 to DesMarais et al. However, such known HIPE polymerization processes disclosed in these patents and other prior art yield materials with large, closed cells which do not contribute to absorption, or must be post-polymerization surface-treated to provide the polymer with hydrophilicity. Furthermore, none of these patents or other prior art disclose polymerizing a monomer or cross-linking a polymer or monomer in an internal aqueous phase.
Notwithstanding these known methods of HIPE polymerization, there is a continuing need to optimize the absorption capacity of low density foams. However, at the same time, there is a continuing need to maintain or even improve other desirable characteristics of these low density foams, such as a capability to absorb both water and oil (hydrophilic and oleophilic properties), structural integrity, and a desirable softness to touch. More absorbent foams would satisfy existing consumer demands for a better product.
It would be desirable to provide a process for making such low density absorbent foams by a HIPE polymerization process where it is possible to minimize the yield of large, closed cells.